U. S. Pat. Nos. 3,470,793; 3,995,534 and 5,302,062 disclose devices for clamping an axially movable rod having spring arrangements for normally urging the rod clamps to one condition and fluid pressure operated means for moving the rod clamps to a second condition. U.S. Pat. Nos. 2,559,925 and 5,314,174 disclose ball joint devices having a ball clamp piston mounted for movement radially of the ball to frictionally hold the ball against movement relative to the socket, with spring means for actuating a piston to one condition and fluid pressure operated means for moving the piston to a second condition. In such ball joint devices, the frictional holding force of the ball clamp piston on the ball is limited by the pressure available for actuating the piston, and difficulties are encountered in installations where the fluid pressure available for actuating the ball clamp piston is low.